It has become common to interconnect a plurality of user digital facilities with a "local area network," i.e., a digital broadcast transmission system located in a restricted geographical area and designed to interconnect the facilities of a single commercial or academic community of users. Such local area networks, LANs, have a variety of different configurations and employ many different transmission protocols. Ethernet.RTM. and token rings are two of the major types of local area networks that are available today. These local area networks (LANs) use various types of digital packets, various signaling protocols and various error detection and correction schemes to insure accurate transmission of digital streams between the users of the local area network. Due to limitations on transmission distance with LAN technology, LAN networks tend to be very restricted in geographical size, typically limited to a university campus or commercial location. Such LAN characteristics are well known and will not be further described here.
It has become increasingly important to interconnect such local area networks together to permit users of different LANs to communicate with each other. Two general types of direct LAN interconnections have been made available, depending on the type of LANs being interconnected. These two interconnection technologies are known as routers and bridges. In general, routers are used to interconnect different configurations of LANs (Ethernet to token ring, for example), over arbitrary distances, while bridges are used to interconnect locally like configurations of LANs (token ring to token ring, for example). Using the Open Systems Interconnection (OSI) model ("Information Processing Systems--Open Systems Interconnection--Basic Reference Model," ISO International Standard 7498, First Edition, Oct. 15, 1984), routers operate at layer 3 of the model (the network layer) while bridges operate at layer 2 (the data link layer, or more precisely, the Medium Access Control (MAC) layer). More specifically, routers at layer 3 terminate the local data link layer protocols and utilize network layer addresses and data frame restructuring to communicate across the WAN interconnection. Layer 3 architectures in which such routers might find use include such well known systems as TCP/IP Internet Protocols (Internetworking with TCP/IP-Principles, Protocols and Architecture, Comer, Prentice-Hall, 1989), OS13 ("Intermediate System to Intermediate System Intra-Domain Routing Protocol for Use in Conjunction with the Protocol for Providing the Connectionless-Mode Network Service," ISO 8473, ISO/DIS 10589, 1990), and SNA ("System Network Architecture--Format and Protocol Reference Manual," IBM Document SC30-3112, 1980).
Bridges at layer 2, on the other hand, utilize the MAC address of the destination from the source LAN itself and thus communicate directly, without frame restructuring, across the interconnection between similar LAN media. That is, since the LANs on both sides of the interconnection use the same protocols, the frames need not be converted at the common LAN interface, but merely delivered across the interconnection from the source LAN to the destination LAN without significant conversion.
More recently, wide area networks, spanning large geographical areas including national and international coverage, have become available for transmitting digital data streams over long distances between users of the system with very high transmission rates, matching, or even exceeding, those of present LANs. Such wide area networks (WANs) have their own transmission, signaling and error handling facilities which generally do not match those of any local area network. It has become a significant problem to transparently interconnect two or more of the LANs described above by means of a WAN backbone in order to permit users on the different LANs to communicate directly with each other. The difficulties in such interconnections are a result, in large part, of the differences in the protocols used by the various LANs, the further different protocols of the WANs themselves, and the widely varying addressing schemes employed by the various LAN protocols.
This proliferation of different protocols for the transmission of digital data packets between geographically separated users has required router style interfaces between the various interconnection systems (LANs and WANs). Such routers, however, are expensive to design and maintain and, moreover, require expensive and time-consuming overhead in order to make all of the high level conversions required at each of the network interfaces. Such overhead interferes severely with the throughput of such systems and significantly increases the cost of interconnecting local area networks by means of the increasingly available wide area networks.